This invention relates generally to surge control in compressor systems, and more particularly to a simple reliable compressor with built-in surge control.
Centrifugal process compressors are subject to flow instability phenomenon, broadly labeled xe2x80x9csurgexe2x80x9d, when the available flow through the compressor impeller drops below a minimum threshold value which is dependent upon various parameters of the individual compressor design.
Continuous operation of a compressor under surge conditions is undesirable as it can damage the compressor. To prevent operation in surge a typical compressor installation includes a surge control or xe2x80x9cfalse loadxe2x80x9d loop of piping, routed from the compressor discharge pipeline, through a control valve back to the compressor inlet piping. During normal operation there is little or no flow through the recirculation pipe loop. When process measurements signal that compressor inlet flow is dropping to levels approaching the onset of surge, a process controller causes the recalculation valve to open sufficiently to allow some of the compressor discharge flow to be reintroduced back at the compressor inlet, thus increasing the flow through the compressor and preventing surge. The design and installation of the surge control-piping loop is a cost consideration at compressor plant sites. Typically this system is not supplied by the compressor manufacturer and thus also requires communication between the plant engineering contractor and the compressor manufacturer to coordinate the operational and space claim requirements of the anti-surge system.
There is need for an improved apparatus that eliminates the requirement for a surge control piping system as described above and which generally allows a reduction in the recirculation mass flow required to prevent surge. There is also need for such an improved apparatus applicable to compressors constructed with machine pressure casings in which the compressor inlet and outlet flow stage pressure boundaries are separated radially by a structural member.
It is a major object of the invention to provide a recirculation or feedback flow loop, contained within the compressor casing, and characterized by a series of passages through the diffuser bracket from the compressor discharge section back to the inlet section and a mechanism for controlling the flow through them.
It is another object to provide for an arrangement of these passages, that can be varied to enhance compressor performance. Adjusting the number, spacing, crossectional area, shape, and angular orientation of these ports to suit individual applications allows the introduction of favorable pre-swirl to the inlet compressor flow. This pre-swirl has the effect of reducing the inlet velocity of the process gas relative to the compressor impeller.
It is yet another object to provide a system that includes
a) a casing having an interior,
b) an impeller rotating within said interior between a fluid supply zone and pressurized fluid discharge zone,
c) fluid flow control members at least one of which is shiftable in the casing interior relative to the other to control fluid back-flow from said discharge zone to said supply zone via through ports in each of said members,
d) and an actuator operatively connected to said one member to shift said one member relative to the other to control the degree of registration of said flow ports in said members, in response to changes in the supply of fluid to said supply zone.
Another object is to provide the members to be relatively rotatable about an axis, and said ports in each member are spaced about said axis. As will be seen, the members preferably extend annularly about that axis, whereby backflow streams through the ports are directed in the supply flow direction, into the impeller inlet streams. In this regard, the ports in at least one member may be elongated in directions substantially parallel to the axis but may also be round; and ports in both members are preferably elongated parallel to that axis for flow control toward the impeller inlet but may also be round and oriented at any angle. Also, ports in each member are preferably spaced at substantially equal intervals about said axis, but may also be spaced at unequal intervals.
An additional object is to locate the impeller entrance to face the supply flow zone and to face toward the incoming backflow streams.
A further object is to provide the casing to include inlet and outlet tubular sections which are interconnected, there being internal structure which is retained in position by the casing inlet and outlet tubular sections, one of said flow control members carried by such internal structure, there being a diffuser at said discharge zone.
A yet further object is to locate the discharge zone from the impeller about the central axis, and also about the registrable ports in the members.